The present invention relates to a flow field plate assembly for fuel cells. More particularly, the invention relates to a flow field plate assembly comprising two flow field plates that engage each other on their inner surfaces such that each inner channel on the inner surface of one plate overlaps an inner channel on the inner surface of the other plate essentially along its entire length. The present invention also relates to a flow field plate assembly comprising two flow field plates, one supporting the other against spreading under compressive load.
Fuel cell systems are currently being developed for numerous applications, such as automobiles and stationary power plants, where they will be used to economically deliver power with significant environmental benefits.
Preferred fuel cell types include solid polymer fuel cells that comprise a solid polymer electrolyte, otherwise referred to as an ion exchange membrane, and operate at relatively low temperatures. The membrane is disposed between two electrodes, namely a cathode and an anode, forming a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst induces the desired electrochemical reactions at the electrodes. During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons pass through an external circuit, creating a flow of electricity.
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
In a fuel cell, these plates on either side of the MEA may incorporate flow fields for the purpose of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. The flow fields comprise fluid distribution channels separated by landings. The channels provide passages for the distribution of reactant to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The landings act as mechanical supports for the fluid diffusion layers in the MEA and provide electrical contact thereto.
Since, during operation, the temperature of the fuel cell may increase considerably and needs to be controlled within admissible limits, flow field plates may also include channels for directing coolant fluids along specific portions of the fuel cell.
An illustration of a fluid flow field plate including coolant fluid channels is described in PCT/International Publication No. WO 00/41260, which is incorporated herein by reference in its entirety. This publication describes a fluid flow field plate in which a major surface is provided with open channels facing the flat surface of another flow field plate. The closed channel formed by the cooperating surfaces of these two plates is used to direct a coolant fluid therethrough.
In an effort to increase and maximize the cross-sectional area of channels in a given plate volume, flow field plates have been contemplated in which channels are incorporated on both surfaces in a corrugated-like fashion. If made of expanded graphite such plates are easy to manufacture and they offer increased thermal and electrical conductivity due to the orientation of the graphite flakes. These plates can have the disadvantage that they may be relatively weak structurally and their channels tend to collapse under compressive load, thereby causing the channel cross-section to change during the operation of the fuel cell.
Expanded graphite sheets, such as those available from UCAR Carbon Technology Corp. (Danbury, Conn., USA) under the tradename GRAFOIL, are advantageously employed as the sheet material from which flow field and separator plates for fuel cells can be formed. In this regard, expanded graphite sheets are well suited for incorporation into fuel cell assemblies, as well as in the manufacture of fuel cell assemblies, particularly because expanded graphite sheets are electrically conductive, chemically stable in fuel cell environments, relatively light, flexible and amenable to low-cost manufacturing methods, such as embossing.
It would be desirable to have a flow field plate assembly comprising two flow field plates, preferably made of expanded graphite, that preserves the advantages of the efficient use of plate volume and the improved thermal and electrical conductivity given by corrugated types of plates while keeping a controlled cross-section for the channels.
It would also be desirable to have a flow field plate assembly that is structurally stronger under compressive load during fuel cell operation thereby preventing deformation of the channels.
A flow field plate assembly comprises two flow field plates that engage each other on their inner surfaces such that the engaged inner channels of each plate define at least one closed inner flow field channel with each inner channel of the second flow field plate overlapping an inner channel of the first field plate essentially along its entire length. A flow field plate assembly, which resists deformation under compressive load, comprises a staggered flow field plate and a corrugated flow field plate.
Herein, the term xe2x80x9coverlappingxe2x80x9d is defined to mean that each inner channel of the second plate is entirely contained within an inner channel of the first plate.
Herein, the term xe2x80x9ccorrugated platexe2x80x9d is defined to mean a plate provided with open inner and outer channels alternating over the planar direction of the plate and having a thickness which is smaller than the sum of the inner and outer channel depths. A xe2x80x9cstaggered platexe2x80x9d is defined to mean a plate provided with open inner and outer channels alternating over the planar direction of the plate whose thickness is greater than the sum of the inner and outer channel depths.
The first flow field plate of the flow field plate assembly is provided with open inner channels on the inner surface of the plate having a width W1 at the inner surface of the plate and open outer channels on the outer surface of the plate. The second flow field plate of the assembly is provided with open inner channels on the inner surface of the plate having a width W2 at the inner surface of the plate smaller than W1, and open outer channels on the outer surface of the plate. The inner surface of the second flow field plate aligns with and engages the inner surface of the first flow field plate such that the engaged inner channels of each plate define at least one closed inner flow field channel with each inner channel of the second flow field plate overlapping an inner channel of the first flow field plate essentially along its entire length. The cross-sectional perimeter of the closed inner flow field channel is the sum of the perimeters of the open inner channels of the first and second flow field plates and the difference (W1-W2) between the widths of the open inner channels.
The outer channels of the two plates of the assembly represent the flow field for the reactants circulated in the fuel cell, fuel (for example, hydrogen or methanol) and oxidant (for example, oxygen or oxygen-containing air). The closed inner flow field is used for circulating coolant fluid in the fuel cell.
Both plates in the flow field plate assembly can be made of a moldable (embossable) material such as, for example, expanded graphite.
The first flow field plate of the above-described assembly can be corrugated and the second flow field plate can be staggered.
A method of making a flow field assembly as described above involves aligning and engaging the inner surface of the first flow field plate with the inner surface of the second flow field plate, such that the engaged inner channels of each plate define at least one closed inner flow field channel with each inner channel of the second flow field plate overlapping an inner channel of the first flow field plate essentially along its entire length.
A flow field plate assembly is also provided that resists deformation under compressive load and comprises a first corrugated plate that tends to spread under compressive load and is therefore supported by a second staggered plate. The corrugated plate, which has open inner channels formed on the inner surface of the plate and open outer channels formed on the outer surface of the plate alternating in the planar direction of the plate, has a thickness smaller than the sum of the inner and outer channel depths. The staggered plate, which has open inner and outer channels formed therein that alternate over the planar direction of the plate, has a thickness greater than the sum of the inner and outer channel depths.
The corrugated and the staggered plate can be made of a moldable (embossable) material such as, for example, expanded graphite.
A method of making a flow field plate assembly as described above involves aligning and engaging a corrugated flow field plate with a staggered flow field plate such that the inner channels of each plate define a closed inner flow field, whereby the staggered plate supports the corrugated plate against spreading under compressive load.
The fuel cell comprising the flow field plate assembly described above is preferably a solid polymer electrolyte fuel cell.